From the Mellotron tape library
http://www.mikedickson.org.uk/tron/under Other Sounds and Services:
" Standard three track tapes have always offered two additional blends at the mid points between A and B and B and C but now you can have 5 unique voices from the new 6 track format. "
Here's what I think:
position A - voice one
position AB - voice two
position B - voice three
position BC - voice four
position C - voice five
You would also be able to dial in four different "mixed voices" between these five selector positions. How bloody marvelous is that?!!